Investigation of Background from the Inter-Spectrometer Penning Trap and Secondary Electron Emission in the KATRIN Experiment

Abstract

The KATRIN experiment aims to determine the neutrino mass with unprecedented precision by measuring the tritium $\beta$-decay spectrum with a tandem of MAC-E-filter spectrometers. The background rate measured at the $\beta$-detector is a crucial parameter affecting the sensitivity of the experiment. The detector energy resolution affects the intrinsic background contribution from the detector system and, therefore, must be well-characterized. Additionally, the increased size of the main spectrometer compared with predecessor experiments necessitates understanding and quantifying various sources of background electrons originating from the spectrometer. Secondary electron emission from the inner spectrometer surface, induced by environmental gamma radiation and cosmic-ray muons, is investigated as a potentially large source of background electrons. A background source unique to the beamline geometry of KATRIN is the inter-spectrometer Penning trap formed during tandem operation of the spectrometers. Detailed measurements and simulations were performed to quantify the background induced by the Penning trap and to confirm the electron-production model. However, by maintaining ultra-high vacuum conditions inside the spectrometers, the effect of the trap on the total background rate is found to be insignificant compared with contributions from other sources.